In the above-referenced parent patent applications, paraformaldehyde is used to react with isobutyraldehyde to make a product comprising in the case of U.S. Ser. No. 691,927, hydroxypivaldehyde (HPA), and in U.S. Ser. No. 716,177, 3-hydroxy-2,2-dimethylpropylhydroxypivalate. The products are hydrogenated to make NPG. The two applications describe different catalyst systems for the aldol reaction; the hydrogenation step is distinguished primarily by the fact that the feed material for it, i.e. the reaction product of the aldol step, after dissolution in a suitable alcohol, may be fed directly to the hydrogenation step. This is due to the use of paraformaldehyde as the formaldehyde reactant, which greatly reduces the presence of water and avoids other complications. The present application is an improvement of both inventions, and in the manufacture of NPG generally, in that the present application recognizes the unique composition of the aldol reaction product made using paraformaldehyde and optimizes the hydrogenation process for it, i.e. for any aldol process utilizing paraformaldehyde. In the present invention, the aldol reaction is not limited to the use of the catalysts described in the parent applications, but embraces any reaction of IBAL and paraformaldehyde; moreover, the hydrogenolysis process of the present invention is applicable to aldol reaction products made with aqueous formaldehyde where the water is substantially removed from the aldol reaction product.
While the present application is limited to the hydrogenation and/or hydrogenolysis of substantially water-free reaction products of IBAL and formaldehyde, preferably paraformaldehyde, certain prior art may be considered relevant wherein HPA or other product obtained from the reaction of IBAL and aqueous formaldehyde is converted to NPG through hydrogenation; such references include those employing copper chromite hydrogenation catalysts, for example. References which employ copper chromite and other hydrogenation catalysts with the conventional aqueous formaldehyde system may be exemplified by U.S. Pat. No. 4,855,515, which recites the historical development of the reaction and emphasizes the use of a particular catalyst in the hydrogenation step. U.S. Pat. No. 3,808,280 discloses the use of triethylamine as a catalyst for the (aqueous) formaldehyde/isobutyraldehyde reaction.
Each of the above references employs formaldehyde in the form of aqueous formaldehyde.
Paraformaldehyde is used by Snam S.p.A. in UK Patent No. 1,017,618 to react with IBAL in the presence of a tertiary amine to produce a reaction product containing apparently predominantly HPA which may be hydrogenated to NPG. However, the instant invention teaches the addition of a suitable alcohol solvent prior to hydrogenation which produces a high purity NPG product by simple distillation, obviating the need for additional expensive purification steps.
While zur Hausen et. al., U.S. Pat. No. 4,250,337 may use the aldol reaction product directly in their hydrogenation step, they also teach the use of small amounts of water, in contrast to the process of the present invention, which advantageously uses an alcohol in the hydrogenolysis step. As a result, our invention achieves high NPG purities together with high yields unlike the aforementioned patent which can only achieve equivalent purities at uneconomical yields.
Other prior art processes which emphasize the hydrogenation step include U.S. Pat. Nos. 4,094,914 to Rottig et. al. and 4,933,473 to Ninomiya et al. Ninomiya et al. especially recognize the formation of the HPA dimer in the aldol reaction product. While Rottig et al. use alcohols in a vapor phase hydrogenation, they do not employ paraformaldehyde in the aldol step as applicants do nor do they recognize or demonstrate ester hydrogenolysis.